Abstract: During the Early Eocene-Early Oligocene rifting period, the Zhuyi depression in the Pearl River Mouth basin mainly experienced acts Ⅰ and Ⅱ of the Zhu-Qiong movement and developed the Wenchang and Enping Formations. Xijiang sag is located in the west of Zhuyi depression. Panyu 4 sag is a proven hydrocarbon-rich depression, while Xijiang main sag and Xijiang 36 sag are not well explored. Based on the latest 3D seismic data and interpretation results, combined with limited drilling data, this paper systematically analyzes the sag structure, fault activity, migration of subsidence center and distribution of depositional system during the Wenchang period and Enping period. It is found that the tectonic transformation during the rift period is mainly reflected in the regular changes of sag structure, fault system, and the migration of subsidence center during the Early Wenchang, Late Wenchang and Enping periods. The migration of the depositional center is consistent with the subsidence center. The subfacies of semi-deep lake and deep lake were developed in the Early and Late Wenchang period, and the depositional center moved southwestern. The depositional center shifted to the north side of Xijiang sag in Enping period. Under the background of clockwise rotation of regional extensional stress direction, the activation of NW trending pre-existing faults in Wenchang led to the transformation of sag structure, fault activity and subsidence center, which controlled the distribution of the depositional system. The tectonic transformation and depositional system distribution of Xijiang depression have a certain significance for searching lacustrine source rocks and hydrocarbon-rich sag in the deep part of Zhuyi depression.

Key words: Pearl River Mouth basin, Xijiang sag, Zhu-Qiong movement, regional stress, tectonic transformation, depositional system